1. Field of the Invention
This invention relates generally to ring pontoon semisubmersible floating platforms, and more particularly to a central pontoon semisubmersible floating platform for use in offshore applications, such as for offshore oil and gas drilling and production, which has a hull with radially oriented columns and a central pontoon structure disposed inboard of the columns that simplifies construction, reduces support spans and cantilevers, and provides improved hydrodynamic performance of the platform.
2. Background Art
Semisubmersible vessels typically have a superstructure deck or decks supported by columns that are attached to hulls or pontoons, which have adjustable ballast capability. By adjusting the ballast carried by the pontoons, the pontoons may be positioned at or near the surface of the water or in a submerged location below the surface of the water, while the superstructure deck remains above the surface of the water. While being moved to a location where the vessel is to be used, the pontoons are typically are de-ballasted to permit them to ride at or near the surface of the water, facilitating transport of the vessel. After reaching the desired location, the ballast of the pontoons may be adjusted to cause the pontoons to become submerged below the surface of the water, providing improved stability and reduced motion of the vessel in rough, deep seas.
In conventional ring-pontoon semisubmersible vessels or platforms, the pontoons support the superstructure deck by columns which rise vertically or substantially vertically from the ring pontoon at various locations, as well as braces which may interconnect the pontoons, the pontoons and the columns, the columns and the superstructure, and/or two or more of the columns, to provide a strong and substantially rigid base structure that supports the deck(s) of the superstructure.
The interior of both the columns and the pontoons may be subdivided by bulkheads to strengthen the structure, to provide enclosed spaces for locating and storing various equipment (e.g., anchors, chains, propulsion mechanisms, etc.), possible storage of liquids such fuel and water, and product storage, and to provide a plurality of separate tanks for purposes of ballasting the vessel and storing various fluids and other materials which may be required or desired during drilling or produced by the well.
In a typical ring pontoon semisubmersible vessel or platform, the corner columns are placed such that the vertical centerline of the columns intersect the axial centerline of the pontoon on which the column is located.
Key et al, U.S. Pat. No. 6,701,861 discloses a semi-submersible floating production vessel which has a ring pontoon with three main columns extending upwardly from corners of the pontoon and three secondary, minor columns extending upwardly from centers of the triangle sides. The columns support an open frame deck, on which production modules are positioned. The vessel is adapted for semi-permanent mooring with pre-tensioned mooring lines that are attached to swivel pad eyes secured on the main columns below the water line. Production and export risers are connected to the vessel below the water line. Compressed air ballast system allows selective emptying of ballast compartments located in the ring pontoon and eliminates the need for a conventional pump room.
Huang et al, U.S. Pat. No. 6,503,023 discloses temporary stability modules and a method for marine structures during construction, transportation and installation that permits the structure, including platform, deck and equipment to be constructed in an upright position, towed to an ocean installation site, and installed by ballasting the structure or temporary stability modules and subsequent removal of the modules. The removable temporary stability modules are shown attached to an “extended-base” “tension leg” platform having four rectangular support columns disposed about a central axis of the substructure and horizontal pontoons interconnecting adjacent columns at their lower ends. The substructure also includes leg extensions radiating from the columns and/or the pontoons, which are described more fully in U.S. Pat. No. 6,447,208, discussed below.
Huang et al, U.S. Pat. No. 6,447,208 discloses an “extended-base” “tension leg” substructure, an offshore platform supported on the substructure and a method for supporting an offshore platform on the substructure, where the substructure includes a plurality of support columns disposed about a central axis of the substructure and interconnected by at least one pontoon. Each column comprises an above-water and submerged portion. The substructure also includes a plurality of wings or arms radiating from the columns and/or the pontoons, each wing fixedly or removably securing at least one tendon extending from a wing to an anchor on the seabed. The substructure includes an open, wave transparent central zone for improved access to well-related equipment, conduits or the like and the wings minimize translational movement and rotational flex in the substructure reducing fatigue in the tendons and their connections.
It should be noted that the U.S. Pat. Nos. 6,503,023 and 6,447,208 are directed toward “extended-base” “tension leg platforms”, which have vertical heave-restrained mooring, wherein the present invention is a semisubmersible structure with lateral spread mooring that is not heave restrained. The outwardly extending wings or extensions of the prior art “extended-base” structures support the tendons some distance outboard of the vertical columns, thus the mooring loads cause restraint against platform vertical and rotational motions. In the present invention, the mooring loads do not provide substantial vertical or rotational restraint, and the vertical columns, which are substantially outboard of the central pontoon, provide improved rotational stability.
Frimm et al, U.S. Pat. No. 6,015,245 discloses a ring pontoon semisubmersible offshore vessel wherein the vertical centerline of each of the corner columns is located inward of both the axial centerline of the forward section of the ring pontoon and the axial centerline of the aft section of the ring pontoon. Additionally, the vertical centerlines of the corner columns may be located inward with respect to the axial centerlines of the starboard and port portions of the ring pontoon. The superstructure deck may be supported by radial braces extending from the ring pontoon to locations on the superstructure deck, which are inward of the ring pontoon. Unlike the present invention, the pontoon structure is not substantially inboard of the columns.
Liden, U.S. Pat. No. 4,498,412 discloses a semi-submersible offshore platform having an operating deck carried by four cylindrical columns supported by a pontoon structure comprising four-sided boxes formed into a square ring. Each pontoon box is subdivided into two compartments by a longitudinal centerline bulkhead, the compartments being further subdivided into tanks by transverse bulkheads. Tanks outside the centerline bulkheads are used for ballast water, and tanks inside the centerline bulkheads are used to store the oil produced. Unlike the present invention, the pontoon structure is not substantially inboard of the columns.
Ludwigson, patent application 20010026733, published Oct. 4, 2001 discloses a semi-submersible vessel adapted to resist heave motion that includes a plurality of support columns with a square-shaped ring-pontoon connected to their lower ends. The axial centerline of the square-shaped ring-pontoon intersects the axial centerline of the lower ends of the support columns, and the support columns are inclined upwardly and inwardly from the ring-pontoon to the deck. Unlike the present invention, the pontoon structure is not substantially inboard of the columns.
The present invention is distinguished over the prior art in general, and these patents in particular by a central pontoon semisubmersible floating platform for use in offshore applications, such as for offshore oil and gas drilling and production, which has a hull configuration including vertical support columns, a central pontoon structure disposed inboard of the columns at a lower end thereof, and a deck structure supported at an upper end of the columns. The vertical columns and pontoon structure are constructed substantially of flat plate. The vertical columns are adjoined to the outer periphery of the central pontoon and have a transverse cross sectional shape with a major axis oriented radially outward from a center point of the hull, and a central vertical axis disposed a distance outward from the pontoon outer periphery. Risers can be supported on the inboard or outboard side of the pontoon and extended to the deck, and the structure can be anchored by mooring lines extending along the outboard face of the columns extending radially outward and downward from their lower ends. The central pontoon and outboard column structure simplifies construction, reduces support spans and cantilevers, and provides improved hydrodynamic performance of the platform.